The present invention relates to a method for controlling an engine connected with a continuously variable transmission (hereinafter referred to as CVT), and more particularly to a method for controlling the RPM of the engine during transient shifting conditions.
A CVT has a V-belt extending between a driving and a driven pulley, the driving and/or driven pulley being able to vary the distance between respective axially movable and non-movable members, thereby controlling the size of their respective V-shaped openings which hold the V-belt. This movement controls the revolution ratio (RPM ratio) between the driving and driven pulleys. The size of the V-shaped opening is directly controlled by the amount of hydraulic pressure being fed into or drained from a hydraulic cylinder of either the driving pulley or the driven pulley. Hence, the effective diameter of the pulleys can be varied by changing the amount of hydraulic pressure being fed into or drained from either of the hydraulic cylinders, thereby allowing for a continuously varying ratio of driving pulley diameter to driven pulley diameter. The V-shaped openings on the driving and driven pulleys are each defined by an area between a non-movable portion of the pulley and a movable portion of the pulley, the size of the V-shaped opening being dependent upon the hydraulic pressure inputted to or drained from the hydraulic cylinders. To provide hydraulic pressure to each of the movable pulleys, a hydraulic apparatus, is provided. The above-described CVT has been used because it exhibits very desirable fuel consumption rates and because the desired horsepower transmitted by the engine-CVT combination is a function of an amount of a throttle opening .theta. exhibited by a throttle valve, a desirable engine RPM is achieved under steady state conditions (RPM.sub.e) such that minimum fuel consumption rates are also achieved.
However, all previously known apparatuses do not adjust the desired engine RPM during transient shifting conditions so that it is different from the desired engine RPM during steady state conditions. Therefore, the amount of time required for the actual engine RPM to obtain the desired engine RPM, when transient shifting conditions are involved, may be undesirably long, thereby impairing the drivability of the vehicle. This impairment is particularly noticeable when the vehicle is required to accelerate. To overcome these disadvantages, a modified method for controlling the engine speed during transient shift conditions was proposed. This modified method utilized the desired engine speed as a function of the amount of throttle opening in a throttle valve. When the throttle valve is opened a large amount, a desired engine speed during a transient shifting condition is determined. This desired engine speed in a transient shifting condition is calculated by multiplying a coefficient "B" by a difference between the desired engine speed under steady state conditions after the change in throttle opening (RPM.sub.os) and the desired engine speed under steady state conditions before the change in throttle opening (RPM.sub.os') i.e., B.(RPM.sub.os -RPM.sub.os'). The coefficient "B" is selected to have a positive value which is less than one. Hence, when the increase in the amount of the throttle opening is large, the engine speed discontinuously increases until it approaches the transient engine RPM, calculated in the above-described manner. After the discontinuous increase in the engine RPM, the engine RPM gradually increases from the transient engine RPM to the desired engine RPM calculated as a function of the amount of change in the throttle opening, RPM.sub.os.
According to the above-described control of the engine RPM, the desired engine speed in the transient shifting condition can smoothly increase. This effects a smooth increase in the acceleration of a vehicle. However, the coefficient B of the above-described control is always a fixed value independent of the velocity of the vehicle or the amount that a throttle valve is opened after the throttle opening has been changed. Hence, the following disadvantages occur. If the coefficient B has a high value suitable for a large vehicle velocity or a large amount of throttle opening after the amount of the throttle opening has changed, the engine speed, RPM.sub.e, does not increase as much when the vehicle operates at a low velocity or when the throttle opening, after the change of the throttle opening, is relatively small. This substantially impairs the acceleration of the vehicle. Conversely, if the coefficient "B" has a low value suitable for a low velocity or a small throttle opening after the change of the throttle opening is made, the amount of increase in the engine RPM from the desired engine RPM before the change in the throttle opening, to the engine RPM in the transient condition becomes small, thereby resulting in a substantial amount of time to reach the desired engine speed after the change of the throttle opening is made. This also substantially impairs the acceleration of a vehicle.